This invention relates generally to electrical circuit components and, more specifically, to vital xe2x80x9cANDxe2x80x9d gates.
As generally understood in the art, a vital component of a system is one configured to fail-safe under credible failure conditions. For example, a vital relay in a control system operating under the closed circuit principle is normally held energized with its front contacts closed. In a fail-safe condition, if a vital relay fails, the front contacts open. Failure, then, of a vital circuit that drives a vital relay de-energizes the relay, resulting in the front contacts of the relay opening. Logic elements such as xe2x80x9cANDxe2x80x9d gates often are required to be vital. Any failure of a vital xe2x80x9cANDxe2x80x9d gate must not result in a permissive, e.g. xe2x80x9conxe2x80x9d, output by the gate.
It is known to use charge pump circuits for driving vital biased-neutral devices, such as relays. Charge pumps are utilized to develop a DC voltage from a pulse train input signal. In a charge pump circuit, input and output voltages can be of opposite polarity. Although known vital electronic circuits that include vital xe2x80x9cANDxe2x80x9d gates generally provide acceptable operational characteristics, they typically include inductive components that are heavy, bulky and expensive. It is also known for high power charge pumps to perform switching using transistors, for example, bipolar transistors along with complicated drive circuitry. Charge pump efficiency is increased if such semiconductor switches have low loss. MOSFPTs have low loss, but when arranged in a charge pump totem pole configuration, exhibit undesirable cross conduction or xe2x80x9cshoot throughxe2x80x9d. To minimize shoot-through time, switching often is performed at high frequencies which are not optimal for some applications.
In one embodiment, a method for performing an xe2x80x9cANDxe2x80x9d operation on two independent vital inputs in a fail-safe manner includes cascading two charge pumps to output a condition signal representing the combined, or AND""ed, state of the vital input devices. Each independent input has an active, or less restrictive, state asserted by a waveform of predetermined frequency and duty cycle. Each independent input also has an inactive, or fail-safe, state asserted by a zero voltage. The method includes supplying power to a first charge pump, supplying power from the output of the first charge pump to a second charge pump, and supplying each of the independent inputs to one of the respective charge pumps. A condition signal is achieved using an output from the second charge pump. More specifically, a DC voltage of a first polarity is asserted to place the output in an active state, or, alternatively, a zero voltage is asserted to place the output in an inactive state. Under certain failure conditions, such as a DC voltage of a second and opposite polarity being output from the second charge pump, a biased neutral device being driven must safely tolerate the failure condition.
Each charge pump receives an independent input signal, e.g. a square waveform generated by one of two independent microcontrollers, or any independently vital means, and the second charge pump output drives a vital biased-neutral device, such as a relay. More specifically, the second charge pump drives the biased-neutral relay only if an independent square waveform is present at the input to each charge pump. One square waveform drives the first charge pump which supplies electrical energy to the second charge pump. The square waveform supplied to the second charge pump generates a voltage that drives the output device. Failure of any charge pump component results in either zero voltage to the output device, or a voltage of polarity opposite to a voltage polarity to which the output device responds. Either condition is considered a fail-safe state.
In another embodiment, the first and second charge pumps are implemented using MOSFET switching components in totem pole configurations. Only one MOSFET at a time is switched on in the charge pump totem pole configuration.
The above-described xe2x80x9cANDxe2x80x9d gate and method provide a high-power, low-loss, and low-cost electrical circuit for operating vital devices responding to specific voltages, for example, vital relays. Because only one MOSFET in each totem pole is on at a time, MOSFET current shoot-through is avoided.